The use of structural adhesives has undergone a substantial increase in the last years, mainly in the aerospace and automotive sectors, with the consequent development of new high-performance adhesive systems. The use of structural adhesives in said sectors has facilitated the replacement of the conventional metallic materials with polymeric materials; furthermore, the generation and development of the class of the structural acrylic adhesives has permitted their application also on non-pretreated surfaces, or even not perfectly clean surfaces, without affecting too much the adhesive performances. The structural acrylic adhesives generally consist of a mixture of acrylic polymers, which polymerize according to a radical mechanism. They exhibit mechanical properties comparable with the ones of the epoxide adhesives, with the advantage of a much shorter setting time, a higher shear and unpact strength and the possibility of realizing resistant joints, even with not very clean surfaces.
Since the acrylic matrices are brittle after cross-linking, the addition of an elastomer to the formulation increases the tenacity thereof; the mechanical properties of the brittle polymers can be inproved, in fact, by incorporating, into the matrix, particles of an elastomeric phase, the presence of which increases the tenacity of the material.
U.S. Pat. No. 3,591,438 describes a two-component composition, in which the adhesive component consists of an acrylic ester, containing a peroxide (as a polymerization starter), and a starter activating agent, consisting of a mixture based on aldehyde-amine condensation products, optionally combined with metal salts. U.S. Pat. No. 3,642,750 describes a sealing composition comprising an acrylic ester, a monovalent cycloaliphatic alcohol and an organic peroxide; another sealing composition is described in U.S. Pat. No. 3,203,941 and is composed of tetraethylene glycol dimethacrylate, of a polyaminic compound and of an organic acid.
In all the cases considered by the prior art, the main components of the adhesive compositions are: a) a sulphur-containing elastomer, selected from chlorosulphonated polyethylene and the mixtures of sulphuryl chloride with chlorinated polyethylene; b) a polymerizable vinyl monomer; c) an organic peroxide as a starter.
Said compositions are not fully free from drawbacks; in fact they exhibit, for example, a low pot life. Furthermore, during their preparation (on an industrial scale )considerable amounts of peroxides or hydroperoxides are to be handled, which, if not handled with the utmost care, can lead to explosions, particularly if the temperature rises. Lastly, the tendency of the peroxides to decompose reduces their capacity of starting the polymerization reaction. U.S. Pat. No. 4,447,588 radically eliminates such problems by utilizing (instead of the peroxides) compounds based on triazine, which are by far stabler than the peroxides or hydroperoxides; said triazinic compounds, however, are difficult to be found and/or synthesized.